Initiation of alkali metal-halogen reaction in combustion systems



UnitedStates Patent 3,508,394 INITIATION OF ALKALI METAL-HALOGEN REACTION IN COMBUSTION SYSTEMS StephenF. De Nagel, Roseville, and Michael T. Tsou,

Farmlngton, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Oct. 25, 1968, Ser. No. 770,524 Int. Cl. F01k 25/08; F03g 7/06 U.S. CI. 60-24 8 Claims ABSTRACT OF THE DISCLOSURE This invention is related to com-bustion reaction systems employed to supply thermal energy to the working gas of a closed cycle thermal engine. More specifically, this invention is related to a technique of initiating and sustaining a combustion reaction between a solid alkali metal fuel charge and a gaseous halogen containing oxidizer in connection with the operation of a closed cycle thermal engine.

US. Patent 3,353,349, issued to Worth H. Percival and assigned to the assignee of this invention, discloses a combustion system for a closed cycle thermal engine. In addition to producing substantial quantities of heat per unit volume or unit weight the combustion system produces nongaseous byproducts and operates at substantially constant volume by employing molten lithium or sodium as the fuel and certain gaseous, nonhydrogen containing Freon-type fluorocarbon compounds as the oxidizer. As disclosed in the Percival patent, which is incorporated herein by reference, the combustion system is particularly useful in closed environments such as underwater applications wherein it is disadvantageous to disposed of a substantial volume of gaseous combustion byproducts. In addition to the Freon-type fluorocarbon oxidizers described in the Percival patent it is also known that other fluorine containing oxidizers such as sulfur hexafluoride and chlorine trifluoride may be reacted with alkali metal fuels to produce liquid or solid combustion reaction byproducts. However, the chemical reaction between alkali metal fuels and fluorine containing oxidizers occurs spontaneously at the metal surface only when the metal fuel has been melted and heated to an elevated temperature, in excess of about 1100' to 1200 F. It is apparent that if a large fuel charge is employed in' a particular application a substantial quantity of energy will be needed to heat the fuel to a suitable reaction temperature.

It is an object of the present invention to provide a technique of initiating and sustaining the combustion reaction between sodium and/ or lithium metal and a suitable fluorine containing oxidizer under conditions which are compatible with the employment of this combustion reaction as a heat source in a closed cycle thermal engine.

It is a more specific object of the present invention to provide a relatively constant volume combustion system for a closed cycle thermal engine, which combustion system may be started up without having to employ an external source of energy to melt the entire fuel metal charge.

In accordance with a preferred embodiment of our in- Patented Apr. 28, 1970 vention these and other objects are accomplished by providing a closed cycle thermal engine, such as a Stirling cycle engine, wherein the Working gas in in heat transfer relationship with an alkali metal fuel charge. In the operation of the closed cycle thermal engine the fuel charge is in the molten state and maintained at an elevated temperature, preferably of about 1200 -1600 F., by reaction with a suitable fluorine containing gaseous oxidizer. However, prior to start up of the engine, the fuel charge is generally solid and relatively cold. To rapidly and efliciently initiate the combustion reaction in accordance with our invention at least one minor portion of the fuel charge is separated from the rest of the charge by a suitable thermal insulating partition. Preferably the insulating partition is a relatively thin walled hemispherical cup formed of an inorganic salt byproduct of the combustion reaction, such as for example lithium fluoride. A relatively small amount of electrical energy is employed to melt only the minor fuel portion and heat it to an elevated temperature at which the molten metal will spontaneously react with a gaseous fluorine containing oxidizer. Once the combustion reaction has been initiated and is self sustaining in the minor fuel portion the heat of combustion melts the insulating partition and heats the rest of the fuel charge. In this manner an eflicient closed cycle thermal engine may be started up without requiring a large quantity of external energy.

The external energy requirements for start up of an engine in accordance with our invention may be further reduced by thermally separating two or more unequal minor fuel portions from the bulk fuel charge, the smaller minor fuel portion being located within the larger minor fuel portion. This is readily accomplished by locating two concentric thin walled hemispherical cups of unequal size within and at the upper surface of the bulk fuel charge. The two insulating cups thus thermally isolate two small portions of the fuel charge, one within the other. The smaller fuel charge is electrically heated to its ignition temperature. Gaseous oxidizer is introduced at the surface of the molten fuel and chemical reaction occurs. The heat of combustion of the smallest fuel charge melts the larger minor fuel portion which ignites, reacts with the available oxidizer and in turn melts and heats all the remaining fuel to a suitable reaction temperature.

These and other objects and advantages of our invention will become more apparent from the detailed description thereof reference being made to the attached drawings in which:

FIGURE 1 is an elevational view in section of a Stirling cycle engine and associated combustion system heat source employing our invention; and

FIGURE 2 is a somewhat enlarged and more detailed view of the upper portion of the metal fuel container of FIGURE 1.

A number of thermal engines have been devised which are of the closed cycle type wherein a working gas is cycled from a relatively high temperautre and pressure vto a relatively low temperature and pressure without leaving the confines of the engine. External heat energy is transferred to the working gas at a relatively high temperature and discharged by the gas at a lower temperature. In general, with respect to engines of this type, the source of thermal energy employed to heat the working gas in one portion of its cycle is more or less independent of the thermal engine itself. A heat source which is suitable for one type of closed cycle thermal engine is likely to be adaptable to others. Thus, it is to be understood that while our invention will be described in the environment of a Stirling cycle engine, it may readily be applicable to engines which operate on other thermodynamic cycles, such as the Brayton cycle and the Rankine cycle.

In FIGURE 1 is illustrated a Stirling type thermal engine in combination with our improved combustion sysem. As shown a Stirling thermal engine has two pisons 12 and 14 which oscillate out of phase in a cylinder t6. For simplicity of illustration only one cyclinder is ;hown in the embodiment of FIGURE 1, but, of course, he engine may comprise two or more cylinders. The lpper piston or displacer piston 12 operates to transfer 1 working gas back and forth between an upper hot space [8 and a lower cold space 20. The lower piston 14 or )ower piston controls the total volume of the system comressing the working gas while it is in the cold space )f cylinder 16 and allowing it to expand while in the :ylinder hot space 18. The working fiuid may be any suittble gas such as hydrogen or helium. As is well known, he expansion of the high temperature high pressure workng gas in the hot space effects the power stroke of the itirling engine. Heater tubes 22 and cooler 24 are conluits in which the working gas is alternately heated and :ooled. Regenerators 26 located between heater tubes 22 1nd cooler 24 store energy when the flow of the working gas is from the top to the bottom and recover energy when the flow is in the opposite direction. Heater tubes 12, which are the situs of thermal energy input to the .ystem, are heated by external means. 28 wherein resides he novel feature of our invention. Power input to the itirling cycle engine is controlled by varying both the teat input and the mean pressure of the working gas. lhe torque and energy transmitted to the power piston [4 during the cycle of the working gas are transferred to I. desired remote place by means of a suitable transmisaion indicated generally at 30.

In the embodiment shown in FIGURE 1 heater tubes 52 are immersed in external heating means 28 in heat :ransmission relationship therewith. This external heating neans comprises a charge of sodium or lithium metal fuel 32 enclosed within a suitable vessel 34. Vessel 34 may 3e filled almost to its capacity with the fuel charge since he combustion reaction products are solid or liquid at :he temperature of the reaction and do not substantially increase the total volume occupied by the reacting system. .VIolten fuel is initially added to vessel 34 preferably unier a protective argon atmosphere after unbolting and removing vessel top 36. In the operation of the engine the ilkali metal fuel is consumed by chemical reaction vith a suitable fluorine containing oxidizer which is atored as a liquid or vapor in container 38 and released is a gas through valve and pressure regulator 40 and :onduit 42 into vessel 34. When the fuel charge has been :onsumed, the hot molten reaction byproducts may, if lesired, be drained from vessel 34 through valve 56.

When engine 10 is not in operation it is preferred to )ermit fuel charge 32 to cool and solidify rather than to naintain it in the molten state. However, the combustion eaction between the fluorine containing oxidizer and the Yuel is sustained only when the fuel is molten and heated o a temperautre of at least about 1l00-l200 F. In orler to initiate the combustion reaction it is apparent that I. solid fuel charge must receive a substantial amount of hermal energy.

One method of supplying the energy to the fuel charge vould be to employ an immersion heater drawing upon an :xternal source of electrical energy. This technique was lescribed in the above-described Percival patent. How- :ver, it is found that for many applications this would 'equire a large source of external energy which might not )e available. For example, it would require about 38,000 vatt-hours to heat one hundred pounds of lithium from F. to its melting point, to melt the lithium, and to heat he molten lithium to 1200 F. We propose a different trrangernent. As shown in FIGURES 1 and 2 minor p01- ions of the fuel charge are separated from the balance by neans of one or more hollow hemispherical cup insulator rartitions 44 and 46. As seen, the partions are of unequal he and are positioned concentrically at the upper surface of the fuel charge, the smaller partition 44 within the larger partion 46. The cups are readily positioned in the fuel charge just after the molten fuel has been poured (cast) into vessel 34 and before the metal has solidified. These partitions are formed of a material which does not conduct heat as readily as does the metallic fuel charge but which will not present difiiculties in the operation of the combustion system. To readily satisfy both requirements it is preferred that partition cups 44 and 46 be formed of an inorganic salt reaction byproduct of the combustion reaction. Examples of such materials include lithium chloride-M.P. 1135 F., lithium fluoride- M.P. 1550 F., sodium chloride-MP. 1480 F. or sodium fluorideM.P. 1820 F. The partition cups are easily stamped or compacted from a quantity of small particles of the inorganic salt.

By employing one or more insulating partitions in a manner like that illustrated, a selected minor portion of the fuel charge may be melted and heated to a suitable reaction temperature with energy from an external source. For example, as shown in FIGURES 1 and 2 the fuel portion 52 contained within the smaller partition 44 is electrically heated by energy supplied through leads 48, the body of the fuel portion 52 providing electrical resistance. Leads 48 are imbedded in the soft metal and insulated from vessel top 36 by insulators 50. Partition 44 retards the transfer of heat from fuel portion 52 to the balance of the fuel and, thus, fuel portion 52 is quickly melted and heated to about 1200 F. Gaseous oxidizer is admitted through regulator valve 40 and duct 42 into. space 54 within vessel 34 above the bulk fuel charge 32': The oxidizer spontaneously reacts with molten fuel portion 52 at the exposed surface thereof whereupon a substantial quantity of heat is released. This heat of combustion then melts partition 54, or is otherwise transferred through it to the balance of the fuel charge.

We have found that the heat of combustion of about 710% by weight of a lithium metal fuel charge, depending upon heat losses from the system 28, is sufiicient to melt and heat the balance of the fuel charge to a suitable spontaneous combustion reaction temperature. Thus, it is necessary to melt and ignite only a minor portion of the fuel charge in the first instance. This may be done by isolating 7-10% of the fuel charge in a single portion (e.g. that within partition 46viz. portions 52 and 58) and heating it by electrical energy from an external source. Alternatively, two or more partitions are employed as shown at 44 and 46 in FIGURES 1 and 2 and a somewhat smaller portion of the fuel charge 52, for example less than about 1% by weight is heated and melted electrically, the heat of combustion of this first small portion 52 being employed to melt and ignite successively larger portions (58 and then the rest of 32) until the en tire mass is in a thermal condition conducive to spontaneous combustion reaction. The advantages of partitioning the bulk fuel charge into two or more minor portions with suitable insulating separators during engine start up becomes more apparent when one considers the following example. Assuming, as suggested above, a bulk fuel charge 32 of one hundred pounds of solid lithium metal, about seven pounds (portions 52 and 58) of the metal would have to be reacted with dichlorotetrafluoroethane, or other suitable oxidizer, to furnish sufiicient heat in situ to melt and heat the rest of the fuel charge. About 2645 watt-hours of electric energy is required to heat seven pounds of lithium metal from 60 F. to 1200 F., neglecting heat losses to the rest of the fuel charge or from the combustion system. If it is desirable to start up a cold engine within a period of two minutes, an external power source capable of providing about 75,000 watts would be required to ignite the seven pound fuel portion. In many instances this would be prohibitively high. However, if a second and smaller portion 52 were isolated, equivalent in weight to about 7% of the first portion (portions 52 and 58) or approximately one-half pound of lithium, only about 8900 watts would be required to heat the one-half pound of lithium to 1200 F. in two minutes. The one-half pound of molten lithium spontaneously ignites with a fluorine containing oxidizer, its heat of combustion provides the heat necessary to melt the surrounding larger minor portion 58 which in turn provides the energy necessary to melt the rest of the fuel charge 32. It is, of course, recognized that in order to initiate the combustion reaction in the manner we propose it is necessary to employ larger fuel charges for a given engine output. In most applications, however, this is preferable to drawing from a large external source of energy.

As indicated in the Percival patent We prefer to employ either lithium or sodium as the fuel charge in our combustion system. They are suitable in all respects for use with a closed cycle thermal engine which is to be started and operated in a closed or submerged environment. Their respective melting points and boiling points are such that they are in the liquid state over the range of temperatures at which most closed cycle thermal engines would be operated. In this respect the melting point of sodium is about 208 F. and its boiling point at one atmosphere pressure is about 1640 F. The melting point of lithium is about 354 F. and its boiling point at one atmosphere of pressure is 2403 F. There are also a number of known and available metals which will contain molten lithium or sodium at temperatures of 1200 F. to 1600 F. or higher for considerable periods of time. Moreover, lithium and sodium form reaction products with the high energy oxidizers preferred for use in accordance with our invention which are solid or liquid at the preferred temperatures of operation. Thus, the combustion system is essentially a low pressure system capable of producing a high temperature of operation.

There are a number of fluorine containing oxidizer compositions which are suitable for use in accordance with our improved combustion system. We prefer to use the Freon-type materials described and claimed in the Percival patent. These oxidizers are Freon-type compounds containing only carbon, fluorine and chlorine in the molecule whereby only graphite and the respective alkali metal halides are produced by reaction with molten sodium or lithium. These preferred oxidizers may be characterized as members of the class C Cl F wherein at is an integer assuming values of at least one, where y and z are integers and the sum of y and z is a value consistent with the value of x in an alkane or cycloalkane type organic compound. As a further restriction 3/ may be equal to zero and z is either equal to or greater than y, but never zero. Examples of suitable Freon-type compounds are dichlorodifluoromethane, dichlorotetrafluoroethane, monochloropentafluoroethane, tetrachlorotetrafluoropropane and trichloropentafluoropropane. Other examples are cited in the Percival patent.

In addition to the preferred Freon-type oxidizers other fluorine containing gases may be employed such as sulfur hexafluoride and chlorine trifluoride. Fluorine gas could also be employed but it is extremely hazardous to handle. These oxidizers will all react with molten lithium and/or sodium to form products which are either solid or liquid at about l2001600 F.

We have described a preferred embodiment of our invention. From this description it is apparent that other forms and embodiments of our invention could be readily adapted by one skilled in the art. Therefore, the scope of our invention is to be considered limited only by the following claims.

We claim:

1. In combination, a closed cycle heat engine; a working gas therein to operate said engine; a combustion system heat source and heat transfer means adapted to transfer heat from said combustion system heat source to said working gas; said combustion system heat source initially comprising a combustion vessel;

a solid metal fuel charge in said combustion vessel, said metal being selected from the group consisting of lithium and sodium;

an oxidizer containing reactive fluorine atoms in the molecule, the composition of said oxidizer being such as to react with said metal fuel charge to form byproducts which are nongaseous below about 1600 F.

oxidizer inlet means adapted to admit said oxidizer into said combustion vessel in the gaseous state in controlled amounts;

a thermal insulator in said combustion vessel in the vicinity of the opening of said oxidizer inlet means into said vessel thermally confining a minor portion of said metal fuel charge from the rest of said metal fuel charge and electric heating means in contact with said minor portion of said metal fuel charge,

whereby said combustion system heat source is adapted for ignition by electrically heating said minor portion of said metal fuel charge to a spontaneous combustion temperature, said oxidizer being operative to spontaneously react with said heated minor portion of said metal fuel charge, the heat of combustion thereof being operative to heat the rest of said metal fuel charge to said spontaneous combustion temperature.

2. In combination, a closed cycle heat engine; a working gas therein to operate said engine; a combustion system heat source and heat transfer means adapted to transfer heat from said combustion system heat source to said working gas; said combustion system heat source initially comprising a combustion vessel;

a solid metal fuel charge in said combustion vessel, said metal being selected from the group consisting of lithium and sodium;

an oxidizer containing reactive fluorine atoms in the molecule, the composition of said oxidizer being such as to react with said metal fuel charge to form byproducts which are nongaseous below about 1600 F.;

oxidizer inlet means adapted to admit said oxidizer into said combustion vessel in the gaseous state in controlled amounts;

an inorganic salt insulator cup, partially spherical in shape, in said combustion vessel in the vicinity of the opening of said inlet means into said combustion vessel thermally confining a minor portion of said metal fuel charge from the rest of said metal fuel charge, said inorganic salt being selected from the group consisting of lithium chloride, lithium fluoride, sodium chloride and sodium fluoride and electric heating means in contact with said minor portion of said metal fuel charge,

whereby said combustion system heat source is adapted for ignition by electrically heating said minor por tion of said metal fuel charge to a spontaneous combustion temperature, said oxidizer being operative to spontaneously react with said heated minor portion of said metal fuel charge, the heat of combustion thereof being operative to melt said inorganic salt insulator cup and to heat the rest of said metal fuel charge to said spontaneous combustion temperature;

3. In combination, a closed cycle heat engine; a working gas therein to operate said engine; a combustion system heat source and heat transfer means adapted to transfer heat from said combustion system heat source to said working gas; said combustion system heat source initially comprising a combustion vessel;

a solid lithium metal fuel charge;

an oxidizer containing reactive fluorine atoms in the molecule, the composition of said oxidizer being such as to react with said metal fuel charge to form byproducts which are nongaseous below about 1600 F.;

' oxidizer inlet means adapted to admit said oxidizer into said combustion vessel in the gaseous state in controlled amounts;

a lithium salt insulator cup, partially spherical in shape,

in said combustion vessel in the vicinity of the opening of said inlet means into said combustion vessel thermally confining a minor portion of said lithium metal fuel charge, up to about 10%, from the rest of said lithium metal fuel charge, said lithium salt being selected from the group consiting of lithium chloride and lithium fluoride and electric heating means in contact with said minor portion of said lithium metal fuel charge,

whereby said combustion system heat source is adapted for ignition by electrically heating said minor portion of said lithium metal fuel charge to a spontaneous combustion temperature, said oxidizer being operative to spontaneously react with said heated minor portion of said lithium metal fuel charge, the heat of combustion thereof being operative to melt said lithium salt insulator cup and to heat the rest of said lithium metal fuel charge to said spontaneous combustion temperature.

4. In combination, a Stirling cycle heat engine, a working gas therein to operate said engine,

a heater tube in said engine in communication with said metal being selected from the group consisting of lithium and sodium;

an oxidizer containing reactive fluorine atoms in the molecule, the composition of said oxidizer being such as to react with said metal fuel charge to form byproducts which are nongaseous below about 1600 F.;

oxidizer inlet means adapted to admit said oxidizer into said combustion vessel in the gaseous state in controlled amounts;

a thermal insulator in said combustion vessel in the vicinity of the opening of said oxidizer inlet means into said vessel thermally confining a minor portion of said metal fuel charge from the rest of said metal fuel charge and electric heating means in contact with said minor portion of said metal fuel charge,

whereby said combustion system heat source is adapted for ignition by electrically heating said minor portion of said metal fuel charge to a spontaneous combustion temperature, said oxidizer being operative to spontaneously react with said heated minor portion of said metal fuel charge, the heat of com' bustion thereof being operative to heat the rest of said metal fuel charge to said spontaneous combustion temperature.

5. In combination, a Stirling cycle heat engine,

a working gas therein to operate said engine,

a heater tube in said engine in communication with said working gas,

a combustionvessel enclosing at least a portion of said heater tube,

a solid metal fuel charge in said combustion vessel,

said metal being selected from the group consisting of lithium and sodium;

an oxidizer containing reactive fluorine atoms in the molecule, the composition of said oxidizer being such as to react with said metal fuel charge to form byproducts which are nongaseous below about 1600 F.;

oxidizer inlet means adapted to admit said oxidizer into said combustion vessel in the gaseous state in controlled amounts;

an inorganic salt insulator cup, partially spherical in shape, in said combustion vessel in the vicinity of the opening of said inlet means into said combustion vessel thermally confining a minor portion of said metal fuel charge from the rest of said metal fuel charge, said inorganic salt being selected from the group consisting of lithium chloride, lithium fluoride, sodium chloride and sodium fluoride and electric heating means in contact with said minor portion of said metal fuel charge,

whereby said combustion system heat source is adapted for ignition by electrically heating said minor portion of said metal fuel charge to a spontaneous combustion temperature, said oxidizer being opera tive to spontaneously react with said heated minor portion of said metal fuel charge, the heat of combustion thereof being operative to melt said inorganic salt insulator cup and to heat the rest of said metal fuel charge to said spontaneous combustion temperature.

6. In combination, a Stirling cycle heat engine,

a working gas therein to operate said engine,

a heater tube in said engine in communication with said working gas,

a combustion vessel enclosing at least a portion of said heater tube,

a solid lithium metal fuel charge in said combustion vessel;

a fluorine containing oxidizer selected from the group consisting of C Cl F wherein x, y and z are integers, x having a value in the range of from one to ten, having a value of zero or greater, z being always equal to or greater in value than y but never zero, and the value of y plus 2 being consistent with the value of x in alkane and cycloalkane type organic compounds,

oxidizer inlet means adapted to admit said oxidizer into said combustion vessel in the gaseous state in controlled amounts;

a lithium salt insulator cup, partially spherical in shape, in said combustion vessel in the vicinity of the opening of said inlet means into said combustion vessel thermally confining a minor portion of said lithium metal fuel charge, up to about 10%, from the rest of said lithium metal fuel charge, said lithium salt being selected from the group consisting of lithium chloride and lithium fluoride, and

electric heating means in contact with said minor portion of said lithium metal fuel charge,

whereby said combustion system heat source is adapted for ignition by electrically heating said minor portion of said lithium metal fuel charge to a spontaneous combustion temperature, said oxidizer being operative to spontaneously react with said heated minor portion of said lithium metal fuel charge, the heat of combustion thereof being operative to melt said lithium salt insulator cup and to heat the rest of said lithium metal fuel charge to said spontaneous combustion temperature.

7. A method of initiating a combustion reaction in a combustion reaction heat source as recited in claim 1 comprising electrically heating said minor portion of said metal fuel charge until said minor portion has been melted and heated to a temperature above about 1100 F., introducing said oxidizer in the gaseous state into said combustion vessel whereby said oxidizer spontaneously reacts with said minor portion of said metal fuel charge and permitting the heat of reaction of said oxidizer with said minor portion of said metal fuel charge to heat the rest of said metal fuel charge to a temperature above about 1100 F.

8,508,394 9 10 8. A method of initiating a combustion reaction in the References Cited apparatus recited in claim 5 comprising electrically heat- UNITED STATES PATENTS ing said minor portion of said metal fuel charge until said minor portion has been melted and heated to a tem- 3,353,349 11/1967 Pel'fiival 6037 perature above about 1100 F., introducing said oxidizer 5 14133301 12/1968 ll et 60 24 in the gaseous state into said combustion vessel whereby 3,418,804 12/1963 Meller et 6O24 said oxidizer spontaneously reacts with said minor portion of said metal fuel charge and permitting the heat MARTIN SCHWADRON Pnmary Exammer of reaction of said oxidizer with said minor portion of R, R. BUNEVICI-I, Assistant Examiner said metal fuel charge to melt said inorganic salt insulator 10 cup and to heat the rest of said metal fuel charge to a US. Cl. X.R.

temperature above about 1100 F. 6037, 59 

